Bimetallic Co–M (M = Cu, Ag, and Au) Carbonyl Complexes Supported by N-Heterocyclic Carbene Ligands: Synthesis, Structures, Computational Investigation, and Catalysis for Ammonia Borane Dehydrogenation

Cristiana Cesari,Rita Mazzoni,Francesco Calcagno,Ivan Rivalta,Beatrice Berti,Carlo Lucarelli,Marco Garavelli,Stefano Zacchini

doi:10.1021/acs.organomet.1c00363

Abstract

The reaction of Na[Co(CO)4] with M(IPr)Cl (M = Cu, Ag, and Au; IPr = C3N2H2(C6H3iPr2)2) affords the neutral heterometallic complexes [Co(CO)4{M(IPr)}] (M = Cu, 1; Ag, 2; and Au, 3). Formation of 2 is accompanied by traces of [Ag(IPr)2][Ag{Co(CO)4}2] (4). The reaction of Na[Co(CO)4] with M(IMes)Cl (IMes = C3N2H2(C6H2Me3)2) results in mixtures of [Co(CO)4{M(IMes)}] (M = Cu, 5; Ag, 6; and Au, 7) and [M(IMes)2][M{Co(CO)4}2] (M = Cu, 8; Ag, 9; and Au, 10). In the cases of Cu and Ag, ionic complexes 8 and 9 are the major products, whereas neutral species 7 is the major product for Au. All species 1–10 have been spectroscopically characterized by IR and 1H and 13C{1H} NMR spectroscopy. Moreover, the molecular structures of 2, 3, and 8 have been determined by single-crystal X-ray diffraction (SC-XRD). Bimetallic Co–M–NHC complexes 1–3 and 7–9 have been tested as catalysts for the dehydrogenation of ammonia–borane (AB) in THF as solvent, and their performances compared to [Fe(CO)4{M(NHC)}2], M(NHC)Cl, and Na[Co(CO)4]. DFT computations have been performed to provide information on the structure, IR spectroscopy, and the thermodynamics of Co–M carbonyl clusters.

Highlights

Heterometallic complexes and clusters represent an emerging class of compounds for the activation of small molecules and catalytic applications.[1−4] Heterobimetallic catalysts of the type (NHC)M−[M′CO] (NHC = N-heterocyclic carbene; M = Cu, Ag, and Au, and ZnCl; [M′CO] = FeCp(CO)[2], Mn(CO)[5], WCp(CO)[3], and Co(CO)4) can be assembled upon mixing M(NHC)Cl and [M′CO]− precursors.[1,5] Because of the polarization of the M−M′ bond of (NHC)M−[M′CO], M is positively charged, and M′ is negatively charged
We report a detailed study on the reactions of Na[Co(CO)4] with M(NHC)Cl (M = Cu, Ag, and Au; NHC = IMes and IPr), which results in heterometallic Co−M complexes. These have been characterized in solution by means of spectroscopic methods (IR and NMR), and their molecular structures are revealed by single-crystal X-ray diffraction (SC-XRD)
The bulkier IPr ligand seems to be the best one in order to stabilize heterometallic Co−M(NHC) bonds, as previously found for related Fe−M(NHC) bonds.[9−13] Such heterometallic Co−M(NHC) and Fe−M(NHC) carbonyl complexes have been tested as catalysts for AB dehydrogenation

Summary

INTRODUCTION

Heterometallic systems containing a coinage metal and a metal carbonyl fragment may be described as metal-only Lewis pairs, since they contain a transition metal Lewis acid (coinage metal) and a transition metal Lewis base (metal carbonyl fragment). We report a detailed study on the reactions of Na[Co(CO)4] with M(NHC)Cl (M = Cu, Ag, and Au; NHC = IMes and IPr), which results in heterometallic Co−M complexes. These have been characterized in solution by means of spectroscopic methods (IR and NMR), and their molecular structures are revealed by single-crystal X-ray diffraction (SC-XRD). The new heterometallic complexes have been tested as catalysts for AB dehydrogenation, and their behavior in solution was further elucidated by means of DFT methods

RESULTS AND DISCUSSION

CONCLUSIONS

EXPERIMENTAL SECTION

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